A mass spectrometer is an analytical device which employs magnetic and/or electrical fields for separating charged particles according to the mass and charge of the particle. The sample material is usually a gas or volatile substance. The sample material is passed at low pressure into the mass spectrometer where it is ionized. Ionization may be accomplished by means of bombardment with a beam of electrons. The energy of the electron beam may be selected to optimize the production of ions having a single positive charge. Ionization can also lead to fragmentation of the sample material. The resulting assortment of ions and ion fragments is then further processed by the mass spectrometer.
The newly created sample ions are accelerated by means of an electric field. The sample ions are then collimated by passage through a series of aligned apertures. The last aperture opens into the mass filter. Once inside the mass filter, sample ions are carried forward by their kinetic energy. However, the trajectories of the sample ions are modified by interactions with the magnetic and/or electric fields which characterize the mass filter. The sample ions are then detected and/or counted as they exit from the mass filter. Often there is an exit slit between the mass filter and the detection means for detecting ions. The exit slit enhances the resolution of the mass spectrum. The resulting mass spectrum consists of a display of the various masses detected and their relative abundances.
The magnetic and/or electric fields of the mass filter may assume a number of configurations. The mass filter may employ a simple magnetic sector with a field perpendicular to the flight path of the sample ions. The trajectory of sample ions traversing the magnetic sector curves according to the mass and velocity of the ion. If good resolution is desired with this type of mass filter, a comparatively large magnet must be employed.
If it is desired to use a relative small magnet while retaining good range and resolution, a cycloidal mass filter may be employed. The cycloidal mass filter employs crossed magnetic and electric fields. The use of crossed magnetic and electric fields reduces the radius of curvature of the trajectory of sample ions as compared to the simple magnetic sector mass filter. Accordingly, a relatively small magnet may be employed with the cycloidal mass filter without sacrificing range or resolution.
On the other hand, if the sample ions have a broad band of kinetic energy, the resolution of the mass spectrometer may by reduced. To correct for situations where there is a broad band of kinetic energy, a Mattauch-Herzog double focusing mass filter may be employed. The Mattauch-Herzog double focusing mass filter employs an initial electrostatic separator for separating ions according to velocity. The sample ions which have passed through the electrostatic separator may then pass through a second set of entry slits which lead to a magnetic separator for separating ions according to mass.
Recently, mass spectrometers have been developed which employ quadrature mass filters. Quadrupole mass filters have an analyzer tube with a four electrode system having both RF and DC voltages. Stable ion trajectories are obtained through the mass filter only if there is a proper relationship between the kinetic energy of the ion and applied RF and DC voltages. Unstable trajectories do not exit from the mass filter. Because the quadrupole mass filter lacks a magnet, it is relatively light and compact and can be scanned relatively quickly. However, the transmission efficiency of the quadrupole mass filter is highly mass dependent.
Employing a time-of-flight method eliminates the need for both a magnetic and an electric field. If the sample ions have uniform kinetic energy and if their entry into the mass filter is pulsed, their exit will also be pulsed, except that ions having different mass will traverse the mass filter with different times of flight. Hence, the output of the ion detector at the exit of the mass filter produces a complete scan of the mass spectrum with each pulse.
Numerous other mass filters applying a variety of separation techniques have been employed. Each mass filter has its advantages and disadvantages. Transmission efficiency of ions through a mass filter to a detector is one characteristic by which all of these various mass filters may be compared.
The absolute transmission efficiency is defined as I/I.sub.0, where I.sub.0 is the number of ions entering the mass spectrometer and I is the number of ions exiting the mass spectrometer, where the mass spectrometer includes all apparatus between the point where an ion is created and the point at the ion detector positioned at the output of the mass filter. A number of factors determine the transmission efficiency. The transmission efficiency is greatly influenced by such factors as instrument geometry and the charge and energy distributions of the sample ions. For example, if both the ingress and egress apertures of the mass filter are relatively wide, the transmission efficiency will be relatively high and the resolution will be relatively low; if both the ingress and egress apertures of the mass filter are relatively narrow, the transmission efficiency will be relatively low and the resolution will be relatively high. Well engineered mass spectrometers will have high values for both the resolution and the transmission efficiency.
Tranmission efficiency may also be mass dependent. For example, the transmission efficiency of the quadrupole filter is highly mass dependent. Such mass filters are sometimes described as discriminating. However, other mass filters, such as the magnetic sector filters, can be designed to have transmission efficiencies which are relatively insensitive to the mass of the sample ions over a broad range. If the transmission efficiency of a mass filter is mass dependent and if the mass filter is to be employed for quantitative measurements, it is essential to know its mass dependent transmission efficiency with some accuracy. Quantitative mass spectroscopy is particularly important when analyzing the chemistry or physics of the ionization process itself.
A method for measuring the relative mass dependent transmission efficiency of mass spectrometers was described by Thomas Ehlert in the Journal of Physics E: Scientific Instruments, Vol. 1, pages 237-239 (1970), "Determination of Transmission Characteristics in Mass Filters." Ehlert's method employs calibrant gases having known isotopic abundances applied at steady partial pressures. Ehlert employed his method to characterize the relative mass dependent transmission efficiency of a quadrupole filter. A modification of Ehlert's method was described and employed by O. J. Orient and S. K. Srivastava for measuring the dissociative electron attachment cross sections for various ion fragments of SO.sub.2. Journal of Chemical Physics, Vol. 78(6), Part I, pages 2949-2952, 15 Mar. 1983, "Production of Negative Ions by Dissociative Electron Attachment to SO.sub.2."
An alternative method for measuring the mass dependent transmission efficiency of mass filters was described by E. Krishnakumar and S. L. Srivastava in the Journal of Physics B: Atomic Molecular and Optical Physics, Vol. 21, pages 1055-1082 (1988), "Ionization Cross Sections of Rare Gas Atoms by Electron Impact." That reference describes a method for measuring the ionization cross sections of gases. The measured ionization cross sections of such gases are then employed for calibrating the transmission efficiency of a mass filter.
The prior-art methods for measuring the transmission efficiency of a mass spectrometer require a knowledge of the electron beam current and either the ionization cross section of calibrant gases or the partial pressure of the unionized sample beam of the calibrant gases. Obtaining this data for calibrant gases and applying these prior-art methods may be difficult and susceptible to error in some cases. What is needed is a simple method and apparatus for determining the absolute transmission efficiency of a mass filter without any knowledge of the electron beam current, the ionization cross section of the calibrant gases and the partial pressure of the unionized sample beam of the calibrant gases.